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  Java Practices -> Serialization and subclassing
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<div class='page-title'>Serialization and subclassing</div>

<div class='main-body'>
 
<br>Interfaces and classes designed for inheritance should rarely implement
<tt><a href="http://java.sun.com/javase/6/docs/api/java/io/Serializable.html">Serializable</a></tt>,
since this would force a significant and (often unwanted) task on their
implementors and subclasses.
<p>However, even though most abstract classes do not implement <tt>Serializable</tt>,
they may still need to allow their subclasses to do so, if desired.
<p>There are two cases. If the abstract class has no state, then it can
simply provide a no-argument constructor to its subclasses. If the abstract
class has state, however, then there is more work to be done, as demonstrated
in the following example.
<p><b>Example</b>
<br>
<PRE>

<span class='keyword'>public</span> <span class='keyword'>abstract</span> <span class='keyword'>class</span> Government {

  <span class='keyword'>public</span> Government( String aWinningParty ) {
    init( aWinningParty );
    <span class='comment'>//never invoke an overridable method in a constructor
</span>  }

  <span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>void</span> raiseTaxes() {
    validateInit(); <span class='comment'>//always called by public methods
</span>    <span class='comment'>//..elided
</span>  }

  <span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>void</span> lowerTaxes() {
    validateInit();
    <span class='comment'>//..elided
</span>  }

  <span class='comment'>/**
  * A template method which calls a protected abstract method, to
  * be overridden by subclasses.
  */</span>
  <span class='keyword'>public</span> <span class='keyword'>boolean</span> makeLaw(){
    validateInit();
    String bill = draftLegislation(); <span class='comment'>//protected, abstract method
</span>    <span class='keyword'>return</span> vote(bill);
  }

  <span class='comment'>// PROTECTED //
</span>
  <span class='comment'>/**
  * Each governament will draft legislation in a particular way.
  * A tyranny, for example, would not have wide debate between its
  * citizens.
  */</span>
  <span class='keyword'>protected</span> <span class='keyword'>abstract</span> String draftLegislation();

  <span class='comment'>/**
  * This no-argument constructor is needed by subclass's Serialization
  * mechanism.
  */</span>
  <span class='keyword'>protected</span> Government() {
    <span class='comment'>//empty
</span>  }

  <span class='comment'>/**
  * Called by both this class's public constructor, and by a subclass's
  * readObject method. Note this method is final.
  */</span>
  <span class='keyword'>protected</span> <span class='keyword'>final</span> <span class='keyword'>void</span> init( String aWinningParty ){
    <span class='keyword'>if</span> (fIsInitialized) {
      <span class='keyword'>throw</span> <span class='keyword'>new</span> IllegalStateException(<span class='literal'>"Cannot call init twice."</span>);
    }
    fWinningParty = aWinningParty;
    fIsInitialized = <span class='keyword'>true</span>;
  }

  <span class='comment'>/**
  * Subclass will need access to the superclass's internal state,
  * either through public or protected gets. Note that the
  * get needs to be final.
  */</span>
  <span class='keyword'>protected</span> <span class='keyword'>final</span> String getWinningParty() {
    <span class='keyword'>return</span> fWinningParty;
  }

  <span class='comment'>// PRIVATE //
</span>  <span class='keyword'>private</span> String fWinningParty;
  <span class='keyword'>private</span> <span class='keyword'>boolean</span> fIsInitialized = <span class='keyword'>false</span>;

  <span class='comment'>/**
  * Must be called by public methods.
  */</span>
  <span class='keyword'>private</span> <span class='keyword'>void</span> validateInit() <span class='keyword'>throws</span> IllegalStateException {
    <span class='keyword'>if</span> (!fIsInitialized) {
      <span class='keyword'>throw</span> <span class='keyword'>new</span> IllegalStateException(<span class='literal'>"Uninitialized object"</span>);
    }
  }

  <span class='keyword'>private</span> <span class='keyword'>boolean</span> vote( String aBill ){
    <span class='keyword'>return</span> <span class='keyword'>true</span>; <span class='comment'>//toy implentation
</span>  }
} 
</PRE>
<br>
<br>
<PRE>

<span class='keyword'>import</span> java.io.*;

<span class='comment'>/**
* A Serializable subclass.
*/</span>
<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> Tyranny <span class='keyword'>extends</span> Government <span class='keyword'>implements</span> Serializable {

  <span class='keyword'>public</span> Tyranny( String aWinningParty ) {
    <span class='keyword'>super</span>( aWinningParty );
  }

  <span class='comment'>// PROTECTED //
</span>
  <span class='keyword'>protected</span> String draftLegislation() {
    <span class='keyword'>return</span> <span class='literal'>"Forbid all use of C++."</span>;
  }

  <span class='comment'>// PRIVATE //
</span>
  <span class='comment'>/**
  * Custom deserialization is needed.
  */</span>
  <span class='keyword'>private</span> <span class='keyword'>void</span> readObject(
    ObjectInputStream aStream
  ) <span class='keyword'>throws</span> IOException, ClassNotFoundException {
    aStream.defaultReadObject();
    <span class='comment'>//manually deserialize and init superclass
</span>    String winningParty = (String)aStream.readObject();
    init( winningParty );
  }

  <span class='comment'>/**
  * Custom serialization is needed.
  */</span>
  <span class='keyword'>private</span> <span class='keyword'>void</span> writeObject(ObjectOutputStream aStream) <span class='keyword'>throws</span> IOException {
    aStream.defaultWriteObject();
    <span class='comment'>//manually serialize superclass
</span>    aStream.writeObject( getWinningParty() );
  }
} 
</PRE>
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<div class='topic-section'>See Also :</div>
<div class='main-body'>
 
  
  <a href='TopicActionb750-2.html?Id=45'>Implementing Serializable</a> <br>
 
  
  <a href='TopicActionaf17-2.html?Id=69'>Designing for subclassing</a> <br>
 
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